Delay circuit having reduced power supply voltage dependency

ABSTRACT

A delay circuit includes: an input signal line (IN) through which an input signal is inputted; a capacitor ( 106 ) charged with and discharging electric charge; a first switch ( 101 ) connected to the input signal line and operating according to the input signal when the capacitor is to be charged with electric charge; a second switch ( 102 ) connected to the input signal line and operating according to the input signal when the electric charge is to be discharged from the capacitor; and a comparison circuit ( 107 ) comparing a voltage of the capacitor and a reference voltage to output a delay signal of the input signal.

This nonprovisional application is a continuation application of andclaims the benefit of International Application No. PCT/JP03/05931 filedMay 13, 2003. The disclosure of the prior application is herebyincorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a delay circuit, and more particularly,to a delay circuit having a capacitor.

BACKGROUND ART

FIG. 9 shows a configuration of a delay circuit according to aconventional art. An input signal line IN is connected to gates of ap-channel MOS transistor 901 and an n-channel MOS transistor 902. Asource of the transistor 901 is connected to a power supply potential,and a source of the transistor 902 is connected to a ground potential. Aresistor 903 is connected between drains of the transistors 901 and 902.They constitute a first-stage inverter.

An output of the first-stage inverter is the drain of the transistor901, which is connected to an input signal line of a second-stageinverter. An output of the second-stage inverter is a drain of atransistor 902, which is connected to an input signal line of athird-stage inverter. An output of the third-stage inverter is a drainof a transistor 901, which is connected to an input signal line of aninverter 905. Capacitors 904 are connected to output signal lines of thefirst-stage to third-stage inverters respectively. A delay signal of aninput signal inputted to the input signal line IN is outputted to anoutput signal line OUT of the inverter 905.

The resistors 903 and the capacitors 904 are capable of adjusting adelay time of the delay signal outputted from the output signal lineOUT. The resistors 903 of the first-stage and third-stage inverters areresistors for adjusting a delay time at the time when the input signalinputted to the input signal line IN rises. The resistor 903 of thesecond-stage inverter is a resistor for adjusting a delay time at thetime when the input signal inputted to the input signal line IN drops.

The delay time of the delay circuit has power supply voltage dependency.The higher the power supply voltage is, the shorter the delay timebecomes. Specifically, when the power supply voltage becomes higher, alarger electric current flows between the sources and drains of thetransistor 901 and 902, so that the switching speed of the transistors901 and 902 becomes higher. As a result, the delay time becomes shorter.

The power supply voltage has been on the decrease in recent years. Thelowered power supply voltage causes the delay time to be longer.Further, with a power supply voltage in a lower voltage range, the delaytime tends to become still longer due to the influence of a thresholdvoltage of the transistors 901 and 902.

In a RAM such as a DRAM or a pseudo SRAM, a predetermined length of timeor longer is constantly necessary for the restore time of memory cells.In a case where the restore time is set in a delay circuit using a highpower supply voltage, the use of a low-power supply voltage results inan excessively long delay time if the power supply voltage dependency ofthe delay time is too large. The excessively long delay time results inan excessively long access time of the RAM determined by the low powersupply voltage. This means that characteristics of the RAM aredetermined by the delay time of the delay circuit.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a delay circuit withreduced power supply voltage dependency.

According to one of the aspects of the present invention, provided is adelay circuit including: an input signal line through which an inputsignal is inputted; a capacitor charged with and discharging electriccharge; a first switch connected to the input signal line and operatingaccording to the input signal when the capacitor is to be charged withelectric charge; a second switch connected to the input signal line andoperating according to the input signal when the electric charge is tobe discharged from the capacitor; and a comparison circuit comparing avoltage of the capacitor and a reference voltage to output a delaysignal of the input signal.

According to the input signal, the first switch operates so that thecapacitor is charged with the electric charge, and the second switchoperates so that the electric charge is discharged from the capacitor.When the capacitor is charged with, for example, a power supply voltage,the time required for discharging the electric charge from the capacitoris longer as the power supply voltage is higher. Since this dischargetime corresponds to the delay time, such a characteristic is exhibitedthat the delay time becomes longer as the power supply voltage ishigher. Conversely, the comparison circuit has a characteristic suchthat its delay time becomes shorter as the power supply voltage ishigher. Consequently, the delay characteristics of the both arecancelled out by each other, so that it is possible to reduce the powersupply voltage dependency of the delay time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration example of a delay circuitaccording to an embodiment of the present invention;

FIG. 2 is a graph showing power supply voltage dependency of delay time;

FIG. 3 is a circuit diagram showing a first example of a more concreteconfiguration of the delay circuit in FIG. 1;

FIG. 4 is a timing chart to describe operations of the delay circuit inFIG. 3;

FIG. 5 is a circuit diagram showing a second example of a more concreteconfiguration of the delay circuit in FIG. 1;

FIG. 6 is a circuit diagram showing a third example of a more concreteconfiguration of the delay circuit in FIG. 1;

FIG. 7 is a block diagram showing a configuration example of asemiconductor memory device in which the delay circuit according to thisembodiment is used;

FIG. 8 is a diagram showing a configuration example of a delay circuitused in a pseudo SRAM or a DRAM; and

FIG. 9 is a diagram sowing a configuration of a delay circuit accordingto a conventional art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a configuration example of a delay circuit according to anembodiment of the present invention. An input signal line IN is a linethrough which an input signal is inputted. A p-channel MOS transistor101 has a gate connected to the input signal line IN, a source connectedto a power supply potential, and a drain connected to a node NA. Ann-channel MOS transistor 102 has a gate connected to the input signalline IN and a drain connected to the node NA. A constant current source(constant current circuit) 103 is connected between a source of thetransistor 102 and a ground potential. Since the transistors 101 and 102constitute an inverter, a logic inverted signal of the input signal ofthe input signal line IN appears at the node NA.

Series connection of a switch 105 and a capacitor 106 is connectedbetween the node NA and the ground potential. When a switch controlsignal is inputted to a terminal 104, the switch 105 is controlled toturn on or off, so that a capacitance value of the capacitor 106 can bechanged. A delay time is determined depending on the capacitance valueof the capacitor 106.

When the input signal inputted to the input signal line IN is at lowlevel (ground potential), the transistor 101 turns on and the transistor102 turns off. When the transistor 101 turns on, a power supply voltageis supplied to the capacitor 106, so that the capacitor 106 is chargedwith the power supply voltage.

Next, when the input signal of the input signal line IN changes to highlevel (power supply potential), the transistor 101 turns off and thetransistor 102 turns on. When the transistor 102 turns on, the electriccharge accumulated in the capacitor 106 is discharged to the groundpotential with a constant current via the constant current source 103.This discharge time “t” is expressed by the following equation. Here,“Q” is an amount of the electric charge of the capacitor 106, “i” isdischarge current, “C” is a capacitance value of the capacitor 106, and“V” is voltage of the capacitor 106.t=Q/i=CV/i

It is understood from this equation that the discharge time “t” islonger as the voltage “V” is higher. This discharge time “t” correspondsto the delay time. Therefore, the higher the power supply voltage withwhich the capacitor 106 is charged, the longer the delay time is.

The voltage of the capacitor 106 is applied to the node NA. A comparator107 compares the voltage of the node NA and a reference voltage V1 tooutput an output signal to a node NB according to the comparison result.Specifically, the comparator 107 outputs high level when the voltage ofthe node NA is higher than the reference voltage V1 while outputting lowlevel when the voltage of the node NA is lower than the referencevoltage V1 (see FIG. 4). FIG. 4 will be described in detail later.Similarly to the node NA, a logic inverted signal of the input signal ofthe input signal line IN appears at the node NB.

The signal of the node NB is inputted to an inverter 108, which thenoutputs a logic inverted signal of this signal to an output signal lineOUT. A delay signal that is the delayed input signal of the input signalline IN is outputted to the output signal line OUT. The delay time ofthe delay signal corresponds to the time from an instant when thetransistor 102 turns on up to an instant when the voltage of the node NAbecomes smaller than the reference voltage V1.

As described above, the signal of the node NA has a delay characteristicsuch that its delay time becomes longer as the power supply voltage ishigher. On the other hand, the comparator 107 has an opposite delaycharacteristic, which will be described later with reference to FIG. 3.Therefore, as the power supply voltage is higher, the delay time of thecomparator 107 becomes shorter since its internal transistors operate athigher speed. As a result, the delay time characteristic of thecapacitor 106 and the delay time characteristic of the comparator 107are opposite to each other to cancel out each other. Power supplyvoltage dependency of the delay signal of the output signal line OUT isreduced.

FIG. 2 is a graph showing the power supply voltage dependency of thedelay time. The horizontal axis shows the power supply voltage and thevertical axis shows the delay time. A characteristic line 201 shows thecharacteristic of the delay circuit according to this embodiment inFIG. 1. A characteristic line 202 shows the characteristic of the delaycircuit in FIG. 9. The characteristic line 202 shows that the delay timebecomes longer as the power supply voltage is lower while the delay timeis shorter as the power supply voltage is higher, and it exhibits asteep change. That is, the power supply voltage dependency of the delaytime is very high. On the other hand, the characteristic line 201 showsthat the power supply voltage dependency of the delay time can bereduced owing to the influence of the comparator 107, though basicallyit exhibits a characteristic such that the delay time is shorter as thepower supply voltage is higher. That is, a difference in the delay timedepending on a difference in the power supply voltage becomes small.

FIG. 3 shows a first example of a more concrete configuration of thedelay circuit in FIG. 1. An input signal line IN is connected to a gateof a p-channel MOS transistor 301 and a gate of an n-channel MOStransistor 302. The transistor 301 corresponds to the transistor 101 inFIG. 1, and the transistor 302 corresponds to the transistor 102 inFIG. 1. The transistor 301 has a source connected to a power supplypotential and a drain connected to a node NA. A resistor 311 isconnected between the node NA and a drain of the transistor 302. Theresistor 311 is not indispensable and may be deleted. An n-channel MOStransistor 303 has a gate connected to a constant potential V2, a sourceconnected to a ground potential, and a drain connected to the source ofthe transistor 302. The transistor 303, which corresponds to theconstant current source 103 in FIG. 1, is capable of supplying aconstant current.

A plurality of sets of series connections of a transistor 305 and acapacitor 306 are connected in parallel. The p-channel MOS transistors305 have gates connected to respective control terminals 304 and sourcesconnected to the node NA. Each of the capacitors 306 is connectedbetween a drain of the transistor 305 and the ground potential. Thecapacitor 306 is constituted of, for example, an n-channel MOStransistor. Specifically, this n-channel MOS transistor has a gateconnected to the drain of the transistor 305, and a source and a drainconnected to the ground potential. When the plural control terminals 304are all set to low level, a capacitance value of the capacitors 306becomes the maximum value. The capacitance value of the capacitors 306can be made larger as the number of the low levels inputted to theplural control terminals 304 is larger. Therefore, a variable capacitorcan be realized. The plural switches 305 correspond to the switch 105 inFIG. 1 and the plural capacitors 306 correspond to the capacitor 106 inFIG. 1.

Next, the configuration of a comparator 307 will be described. Thecomparator 307 has a current mirror circuit and corresponds to thecomparator 107 in FIG. 1. Hereinafter, the configuration of the currentmirror circuit will be described. A p-channel MOS transistor 322 c has agate connected to a node NC, a source connected to a power supplypotential, and a drain connected to the node NC. An n-channel MOStransistor 323 c has a gate connected to the node NA, a drain connectedto the node NC, and a source connected to a node ND. A p-channel MOStransistor 322 b has a gate connected to the node NC, a source connectedto the power supply potential, and a drain connected to the node NB. Ann-channel MOS transistor 323 b has a gate connected to a referencepotential V1, a drain connected to the node NB, and a source connectedto the node ND. An n-channel MOS transistor 324 has a gate connected toan output terminal of a negative-AND (NAND) circuit 314, a drainconnected to the node ND, and a source connected to the groundpotential. An inverter 308 has an input terminal connected to the nodeNB and an output terminal connected to an output signal line OUT.

Next, a configuration of a reset circuit will be described. An inverter312 has an input terminal connected to the output signal line OUT and itoutputs a logic inverted signal. An inverter 313 has an input terminalconnected to the input signal line IN and it outputs a logic invertedsignal. An output signal of the inverter 313 and an output signal of theinverter 312 are inputted to the NAND circuit 314, which then outputs anegative logical product signal. A p-channel MOS transistor 321 c has agate connected to the output terminal of the NAND circuit 314, a sourceconnected to the power supply potential, and a drain connected to thenode NC. A p-channel MOS transistor 321 b has a gate connected to theoutput terminal of the NAND circuit 314, a source connected to the powersupply potential, and a drain connected to the node NB.

FIG. 4 is a timing chart to describe operations of the delay circuit inFIG. 3. The input signal line IN and the output signal line OUT are atlow level (0 V) before a time t1. Since the input signal line IN is atlow level, an output signal of the inverter 313 is at high level. Sincethe output signal line OUT is at low level, an output signal of theinverter 312 is at high level. Therefore, the NAND circuit 314 outputs alow-level output signal. As a result, the transistors 321 c and 321 bturn on and the transistor 324 turns off. At this time, the comparator307 does not make the comparison and the node NB becomes at the powersupply voltage (high level) Vcc irrespective of the potential of thenode NA. Since the node NB is at high level, the inverter 308 outputslow level to the output signal line OUT. At this time, the transistor324 turns off to bring the comparator 307 into a deactivated state, sothat no electric current flows, which can reduce power consumption.

When the input signal line IN is at low level, the transistor 301 turnson and the transistor 302 turns off. As a result, a power supply voltageVcc is supplied to the capacitors 306, so that the capacitors 306 arecharged with the power supply voltage Vcc. The node NA becomes at thepower supply voltage Vcc.

It is assumed that at the time t1, the input signal inputted to theinput signal line IN next rises from low level to high level. When theinput signal line IN becomes at high level, the NAND circuit 314 outputshigh level. This causes the transistors 321 c and 321 b to turn off andthe transistor 324 to turn on, so that the comparator 307 is broughtinto an activated state.

When the input signal line IN becomes at high level, the transistor 301turns off and the transistor 302 turns on. As a result, the electriccharge is discharged from the capacitors 306, so that the constantcurrent flows through the resistor 311 and the transistors 302 and 303.The voltage of the node NA gradually lowers from the power supplyvoltage Vcc. This voltage drop time of the node NA corresponds to thedelay time. The higher the power supply voltage Vcc is, the longer thedelay time is.

Next, operations of the current mirror circuit will be described. Thetransistor 324 functions as a constant current source. When the voltageof the node NA is higher than the reference voltage V1, a large electriccurrent flows through the transistor 323 c and a small electric currentflows through the transistor 323 b. Conversely, when the voltage of thenode NA is lower than the reference voltage V1, a large electric currentflows through the transistor 323 b and a small electric current flowsthrough the transistor 323 c. The sum of a value of the electric currentflowing through the transistor 323 b and a value of the electric currentflowing through the transistor 323 c is a constant value. Since thegates of the transistors 322 b and 322 c are both connected to the nodeNC, the same electric current can flow through the transistors 322 b and322 c, so that they function as the current mirror. As a result, whenthe voltage of the node NA becomes lower, the voltage of the node NBalso becomes lower, and in due time, the output signal line OUT of theinverter 308 rises from low level to high level. At the output signalline OUT, the delay signal of the input signal of the input signal lineIN appears.

Thereafter, when the input signal of the input signal line IN becomes atlow level, the output signal of the output signal line OUT is delayed toalso become at low level. As described above, when the input signal lineIN and the output signal line OUT both become at low level, the outputsignal of the NAND circuit 314 becomes at low level, the node NB isfixed at high level, and the output signal line OUT becomes at lowlevel.

As described above, the comparator 307 has the current mirror circuit.As the power supply voltage is higher, the transistors in the currentmirror circuit operate at higher speed since their source-drain currentbecomes larger. As a result, the delay time in the comparator 307becomes shorter as the power supply voltage is higher.

In the node NA, the delay time becomes longer as the power supplyvoltage is higher. Conversely, in the comparator 307, the delay timebecomes shorter as the power supply voltage is higher. As a result, thedelay characteristics of the both are canceled out by each other toreduce the power supply voltage dependency of the delay time.

FIG. 5 shows a second example of a more concrete configuration of thedelay circuit in FIG. 1. What are different in the delay circuit in FIG.5 from the delay circuit in FIG. 3 will be described. This delay circuitdoes not have the reset circuit in FIG. 3, but may have the resetcircuit. Transistors 305 a and 305 b correspond to the transistors 305in FIG. 3, and capacitors (transistors) 306 a and 306 b correspond tothe capacitors (transistors) 306 in FIG. 3.

The p-channel MOS transistors 305 a have gates connected to controlterminals 304 via inverters 501 respectively and sources connected to anode NA. The n-channel MOS transistors 305 b have gates connected to thecontrol terminals 304 respectively, drains connected to the node NA, andsources connected to drains of the transistors 305 a respectively.

Each of the n-channel MOS transistors 306 a has a gate connected to thetransistors 305 a and 305 b, and a source and a drain connected to aground potential. Each of the p-channel MOS transistors 306 b has a gateconnected to the transistors 305 a and 305 b, and a source and a drainconnected to a power supply potential.

Thus, the switch 105 and the capacitor 106 in FIG. 1 can be constitutedusing only n-channel MOS transistors, only p-channel MOS transistors, orboth of them. In a case where the switch 105 is constituted only of then-channel MOS transistors, by inputting a voltage larger than the powersupply voltage to the gates of these transistors, on-resistance of thetransistors can be reduced.

An n-channel MOS transistor 521 has a gate connected to a constantpotential V2, a drain connected to a node ND, and a source connected tothe ground potential. The transistor 521, which functions as a constantcurrent source, is capable of controlling a constant current value by aconstant potential V2.

FIG. 6 shows a third example of a more concrete configuration of thedelay circuit in FIG. 1. What are different in the delay circuit in FIG.6 from the delay circuit in FIG. 3 will be described. This delay circuitdoes not have the reset circuit in FIG. 3 but may have the resetcircuit. When the reset circuit is not provided, a gate of a transistor324 is connected to a power supply potential.

In this delay circuit, a resistor 611 is connected instead of theresistor 311 in FIG. 3, and the transistor 303 in FIG. 3 is deleted. Theresistor 611 is a fixed resistor whose resistance value is large andfunctions as a constant current source. Since the resistor 611 functionsas the constant current source, the transistor 303 in FIG. 3 is notrequired.

FIG. 7 shows a configuration example of a semiconductor memory deviceusing the delay circuit according to this embodiment. This semiconductormemory device is a pseudo SRAM (static random access memory). The pseudoSRAM has memory cells with a DRAM (dynamic random access memory)structure and has a built-in refresh circuit that automatically andperiodically refreshes the memory cells.

An address ADR and a command CMD can be externally inputted thereto, anddata DT can be inputted/outputted thereto/therefrom. For example, for aread operation, a read command as the command CMD and a read address asthe address ADR are inputted, and it can read data from a memory cellarray 708 and output the read data as the data DAT. For a writeoperation, a write command as the command CMD, a write address as theaddress ADR, and write data as the data DT are inputted, and it canwrite the data to the memory cell array 708.

A command decoder 703 decodes the command CMD to output a control signalto an address latch 704 and a timing generator 705. According to thecontrol signal, the address latch 704 latches the address ADR to outputa row address to a row decoder 706 and output a column address to acolumn decoder 707. The row decoder 706 decodes the row address and thecolumn decoder 707 decodes the column address. The memory cell array 708has a large number of memory cells arranged two-dimensionally. Each ofthe memory cells can store data. Based on the decoding by the rowdecoder 706 and the column decoder 707, memory cells of, for example, 16bits are selected by word lines.

The timing generator 705 generates a timing signal according to theinputted control signal to output it to a sense amplifier 709. The senseamplifier 709 reads the data from the selected memory cells andamplifies the data to output it as the data DT. Further, the senseamplifier 709 writes the data inputted as the data DT to the selectedmemory cells.

A refresh controller 702 periodically outputs a refresh command RQ tothe command decoder 703 based on an oscillation signal generated by anoscillator 701 and outputs a refresh address to the address latch 704.Consequently, the sense amplifier 709 amplifies the data in the selectedmemory cells to restore (write back) the data to the memory cells.

FIG. 8 shows a configuration example of a delay circuit used in a pseudoSRAM or a DRAM. This delay circuit has the same basic configuration asthat in FIG. 1 but additionally has a circuit supplying a signal to acontrol terminal 104. This delay circuit can set different delay timesin a refresh operation, a read operation, and a write operation. Forexample, it can set the delay time for setting the time from the rise ofvoltage of a word line up to activation of a sense amplifier, or thedelay time for setting the time from the activation of the senseamplifier to the drop of the voltage of the word line.

A command decoder 815 decodes a command CMD to output an operation modesignal CTL. The command CMD and the operation mode signal CTL includecommands and operation mode signals for refresh, read, and writerespectively. A selector 816 selects, according to the operation modesignal CTL, delay time information outputted by one of a refresh delaytime setting circuit 801, a read delay time setting circuit 802, and awrite delay time setting circuit 803 to output the selected informationto the control terminal 104. The selector 816 selects the refresh delaytime setting circuit 801 when the operation mode signal CTL is a refreshoperation signal, selects the read delay time setting circuit 802 whenthe operation mode signal CTL is a read operation signal, and selectsthe write delay time setting circuit 803 when the operation mode signalCTL is a write operation signal.

A configuration of the refresh delay time setting circuit 801 will bedescribed. A test command decoder 811 decodes an inputted test code TCto output delay time information S1 and a test mode signal TM to aselector 814. The test code TC is inputted in a test mode, so that adesired delay time can be set.

A fuse circuit 812 is a memory storing the delay time information forthe refresh operation and can store information by fuse connection orfuse blowout. The decoder 813 decodes the delay time informationoutputted by the fuse circuit 812 to output delay time information S2.In a normal operation mode other than the test mode, the delay timeinformation S2 is used.

The selector 814, when the test mode signal TM is inputted thereto,selects the delay time information S1, and in other cases, selects thedelay time information S2 to output refresh delay time information S11.

The read delay time setting circuit 802 and the write delay time settingcircuit 803 have the same configuration as that of the refresh delaytime setting circuit 801. The read delay time setting circuit 802outputs delay time information S12 for the read operation. The writedelay time setting circuit 803 outputs delay time information S13 forthe write operation.

The selector 816 outputs one of the delay time information S11, S12, andS13 as delay time information S20 to the control terminal 104 accordingto the operation mode signal CTL. A capacitance value of a capacitor 106is set according to the delay time information S20 as described above.

As described above, the capacitance value of the variable capacitor 106is variable according to the input of the test code TC, is variableaccording to the information stored in the fuse circuit 812, and isvariable according to the operation mode signal CTL.

In a RAM such as a pseudo SRAM or a DRAM, a predetermined length of timeor longer is constantly necessary for the restore time of memory cells.This restore time can be set by the delay time of the delay circuit.Reducing power supply voltage dependency of the delay time can reducepower supply voltage dependency of the restore time and can prevent theaccess time of the RAM from becoming longer than necessary. That is, itis possible to reduce variation in the access time of the RAM dependingon the power supply voltage and to shorten a prescribed value of theaccess time of the RAM.

According to this embodiment, the transistor 101 in FIG. 1 is a switchconnected to the input signal line IN and operating according to theinput signal when the capacitor 106 is to be charged with electriccharge. The transistor 102 is a switch connected to the input signalline IN and operating according to the input signal when the electriccharge is to be discharged from the capacitor 106. The transistors 101and 102 are switches performing opposite operations according to theinput signal. The comparison circuit includes the comparator 107 and theinverter 108 and compares the voltage of the capacitor 106 and thereference voltage V1 to output the delay signal of the input signal.

When the capacitor is charged with, for example, the power supplyvoltage, it takes a longer time to discharge the electric charge fromthe capacitor as the power supply voltage is higher. Since thisdischarge time corresponds to the delay time, such a characteristic isexhibited that the delay time becomes longer as the power supply voltageis higher. Conversely, the comparison circuit exhibits a characteristicsuch that its delay time becomes shorter as the power supply voltage ishigher. Consequently, the delay characteristics of the both arecancelled out by each other, so that power supply voltage dependency ofthe delay time can be reduced.

It should be noted that the above-described embodiments only showexamples of implementing the present invention, and these embodimentsshould not be interpreted as limiting the technical scope of the presentinvention. Therefore, the present invention can be implemented invarious forms without departing from the technical spirit and majorcharacteristic thereof.

INDUSTRIAL APPLICABILITY

According to an input signal, a first switch operates so that acapacitor is charged with electric charge and a second switch operatesso that the electric charge is discharged from the capacitor. When thecapacitor is charged with, for example, a power supply voltage, it takesa longer time to discharge the electric charge from the capacitor as thepower supply voltage is higher. Since this discharge time corresponds tothe delay time, such a characteristic is exhibited that the delay timebecomes longer as the power supply voltage is higher. Conversely, acomparator has such a characteristic that its delay time is shorter asthe power supply voltage is higher. Therefore, since the delaycharacteristics of the both are cancelled out by each other, it ispossible to reduce power supply voltage dependency of the delay time.

1. A delay circuit comprising: an input signal line through which aninput signal is inputted; a capacitor charged with and dischargingelectric charge; a first switch connected to said input signal line andoperating according to the input signal when said capacitor is to becharged with electric charge; a second switch connected to said inputsignal line and operating according to the input signal when theelectric charge is to be discharged from said capacitor; a comparisoncircuit comparing a voltage of said capacitor and a reference voltage tooutput a delay signal of the input signal; and a reset circuit resettinga level of an output signal line of said comparison circuit according tothe input signal and the delay signal.
 2. The delay circuit according toclaim 1, wherein said first and second switches perform oppositeoperations according to the input signal.
 3. The delay circuit accordingto claim 1, further comprising a constant current source discharging theelectric charge from said capacitor with a constant electric currentaccording to the operation of said second switch.
 4. The delay circuitaccording to claim 1, wherein a power supply voltage is supplied to saidcapacitor through said first switch to charge said capacitor.
 5. Thedelay circuit according to claim 1, wherein said comparison circuitincludes: a comparator comparing the voltage of said capacitor and thereference voltage; and an inverter inverting logic of an output of thecomparator.
 6. The delay circuit according to claim 5, wherein saidcapacitor is a variable capacitor and a delay time of the delay signalis adjustable according to a capacitance value of said capacitor.
 7. Thedelay circuit according to claim 6, wherein said variable capacitor hasa plurality of sets of a third switch and a capacitor.
 8. The delaycircuit according to claim 7, wherein said capacitor is constituted ofan n-channel transistor and/or a p-channel transistor.
 9. The delaycircuit according to claim 8, wherein the third switch is constituted ofan n-channel transistor and/or a p-channel transistor.
 10. The delaycircuit according to claim 1, wherein said first switch is a p-channeltransistor, and said second switch is an n-channel transistor.
 11. Thedelay circuit according to claim 1, wherein said comparison circuitcomprises a current mirror circuit.
 12. The delay circuit according toclaim 1, wherein said comparison circuit makes the comparison after theinput signal rises.
 13. The delay circuit according to claim 1, whereinsaid reset circuit supplies high level to an output signal line of saidcomparison circuit when the input signal and the delay signal are at lowlevel.
 14. The delay circuit according to claim 3, wherein said constantcurrent source is a circuit constituted of an n-channel transistorhaving a gate to which a fixed voltage is applied.
 15. The delay circuitaccording to claim 3, wherein said constant current source isconstituted of a resistor.
 16. The delay circuit according to claim 6,wherein the capacitance value of said variable capacitor is variableaccording to an input of a test code.
 17. The delay circuit according toclaim 6, wherein the capacitance value of said variable capacitor isvariable according to information stored in a memory.
 18. The delaycircuit according to claim 6, wherein the capacitance value of saidvariable capacitor is variable according to an operation mode.
 19. Thedelay circuit according to claim 18, wherein the operation mode is oneof a refresh operation mode, a read operation mode, and a writeoperation mode of a semiconductor memory device.
 20. A delay circuitcomprising: an input signal line through which an input signal isinputted; a capacitor charged with and discharging electric charge; afirst switch connected to said input signal line and operating accordingto the input signal when said capacitor is to be charged with electriccharge; a second switch connected to said input signal line andoperating according to the input signal when the electric charge is tobe discharged from said capacitor; a comparison circuit comparing avoltage of said capacitor and a reference voltage to output a delaysignal of the input signal; a constant current source discharging theelectric charge from said capacitor with a constant electric currentaccording to the operation of said second switch; and a resistorconnected between said capacitor and said constant current source.